Complement-regulatory protein expression and activation of complement cascade on erythrocytes from patients with rheumatoid arthritis (RA).

It has been previously reported that the expression of the complement receptor, CR1, on erythrocytes is reduced in patients with RA and that the reduced expression of CR1 is related to disease activity. In this study we investigate the role of other regulatory proteins, i.e. decay-accelerating factor (DAF) and CD59 (membrane inhibitor of reactive lysis), in the pathogenesis of RA by checking the expression of DAF and CD59 on erythrocytes of RA patients to establish whether reduced expression of DAF and CD59 on erythrocytes could be related to increased ability of erythrocytes to activate complement in RA. Flow cytometry was used to measure the expression of DAF and CD59 on erythrocytes from RA patients as well as the deposition of C3 fragments occurring in vivo or after in vitro complement activation. Significantly reduced expression of DAF and CD59 was observed on erythrocytes of RA patients. A significant inverse relationship was observed between DAF expression and in vitro complement activation, whereas no significant relationship between CD59 and complement activation was observed. Finally, we demonstrated an inverse relationship between CH50 activity and DAF expression. Thus, determination of DAF on erythrocytes can emerge as an additional tool in the assessment of extent of complement activation in RA.